As a positive-working photoresist composition there is normally used a composition comprising an alkali-soluble resin binder such as novolak and a naphthoquinonediazide compound as a photosensitive material.
A novolak resin which serves as a binder can be dissolved in an alkaline aqueous solution without swelling. The novolak resin can also exhibit a high resistance particularly to plasma etching when an image thus produced is used as a mask for etching. Thus, novolak resin is particularly useful in this application. A naphthoquinone diazide compound which serves as a photosensitive material itself serves as a dissolution inhibitor for reducing the alkali solubility of novolak resin but is peculiar in that it undergoes decomposition upon the irradiation with light to produce an alkali-soluble substance which rather enhances the alkali solubility of novolak resin. Because of the great change in properties by the irradiation with light, naphthoquinone diazide compound is particularly useful as photosensitive material for positive-working photoresist.
From such a standpoint .of view, many positive-working photoresists comprising novolak resin and naphthoquinone photosensitive material have heretofore been developed and put into practical use. These positive-working photoresist materials have showed marked progress particularly towards high resolving power and attained sufficient results in working lines having a width as small as submicron.
It has heretofore been considered that a resist having a high contrast (.gamma. value) can be advantageously used to provide reproduction of a faithfully-patterned image with an enhanced resolving power. In an attempt to accomplish the foregoing object, various resist compositions have been developed. An extremely large number of publications disclose this technique. Referring particularly to novolak resin as a main component of positive-working photoresist, many monomer compositions, molecular weight distributions, synthesis methods, etc. have been applied for patent. These patents have attained some good results.
An attempt to improve the properties of resists by providing a novolak resin with a specified molecular weight distribution is known. For example, JP-A-1-105243 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses that a novolak resin having a molecular weight distribution such that the molecular weight range of 500 to 5,000 accounts for not more than 30% of the molecular weight distribution is desirable. Further, JP-A-62-227144 and JP-A-63-2044 disclose that there is a desirable proportion of specified molecular weight ranges in the molecular weight distribution. Moreover, JP-A-60-97347 and JP-A-60-189739 disclose the use of a novolak resin from which a low molecular weight component has been fractionally removed. Also, JP-A-60-45238 disclose the use of a resin having a dispersion degree of not more than 3 as used herein.
Referring to photosensitive material as another main component of positive-working photoresist, many compounds structured so as to provide a high contrast have been disclosed. The design of positive-working photoresist taking advantage of these techniques makes it possible to develop a ultrahigh resolving resist that can resolve a pattern with the same dimension as the wavelength of light.
However, integrated circuits have developed its degree of integration more and more. In the production of semiconductor substrates such as SLSI (super large scale integration), the working of ultrafine patterns composed of lines having a width down to not more than 0.5.mu.m has been required. Such an application has required a photoresist having a development latitude wide enough to provide an invariably high resolving power and keep the width of worked lines constant. In order to prevent circuits from being malworked, it is necessary that no resist residues occur on the resist pattern developed.
Further, a low molecular compound having aromatic hydroxyl group is normally used as a dissolution accelerator or for the purpose of enhancing sensitivity. Many cases of the addition of such a low molecular compound have been disclosed. However, the addition of such a compound normally causes a rise in the reduction of the thickness of unexposed area, resulting in the deterioration of the shape of the resist. In addition, the addition of such a compound increases the development speed and hence reduces the development latitude. Thus, the structure of desirable compounds have been selected such that these phenomena can be minimized.
An attempt to improve the sensitivity and developability of resists by blending a specific compound in the resist compositions has been made. For example, JP-A-61-141441 discloses a positive-working photoresist composition containing trihydroxybenzophenone. The positive-working photoresist composition containing trihydroxybenzophenone can exhibit an improved sensitivity and developability but is disadvantageous in that the presence of trihydroxybenzophenone causes a deterioration of the heat resistance that results in the rise in the reduction of the film thickness of unexposed area.
It has also been found that the formation of an ultrafine pattern having a line width as small as not more than 0.5.mu.m is liable to a phenomenon in which, although a specified resolving power can be obtained with a predetermined coat film thickness, even a slight change in the coat film thickness can deteriorate the resolving power (hereinafter referred to as "dependence on film thickness"). A surprising fact has also been found that even a change in the film thickness as small as few hundredths of micron can cause a great change in the resolving power and any commercially available typical positive-working photoresists show such a behavior more or less. In some detail, when the thickness of the resist film which has not been exposed changes in a proportion of .lambda./4n (in which .lambda. is the wavelength of light to which the resist is exposed, and n is the refractive index of the resist film at the same wavelength), the resulting resolving power can vary.
Referring to the problem of dependence on film thickness, its presence is noted in, e.g., SPIE Proceedings, vol. 1,925, page 626, 1993. The paper reports that this phenomenon can be caused by a multiple light reflection effect in the resist film.
It has been found that this dependence on film thickness becomes remarkable particularly when the contrast of the resist is raised such that a high resolving power and a pattern having a rectangular section can be obtained. In the actual work of semiconductor substrates, a pattern is formed using a resist film having slight fluctuation in film thickness due to roughness on the surface of the substrate or unevenness in the coat film thickness. Thus, this dependence on film thickness is one of obstacles to the work of ultrafine patterns close to the limit of resolution with a positive-working photoresist.
Many 1,2-naphthoquinonediazidization products of polyhydroxy compound having a specific structure have heretofore been proposed to enhance the resolving power. These compounds are disclosed in JP-A-57-63526, JP-A-60-163043, JP-A-62-10645, JP-A-62-10646, JP-A-62-150245, JP-A-63-220139, JP-A-64-76047, JP-A-1-189644, JP-A-2-285351, JP-A-2-296248, JP-A-2-296249, JP-A-3-48249, JP-A-3-48250, JP-A-3-158856, JP-A-3-228057, JP-A-4-502519, U.S. Pat. Nos. 4,957,846, 4,992,356, 5,151,340, and 5,178,986, and EP 530,148. However, even these photosensitive materials leave something to be desired in the reduction of dependence on film thickness.
On the other hand, it is disclosed in JP-B-37-18015 (The term "JP-B" as used herein means an "examined Japanese patent publication"), JP-A-58-150948, JP-A-2-19846, JP-A-2-103543, JP-A-3-228057, JP-A-5-323597, EP 573,056, U.S. Pat. Nos. 3,061,430, 3,130,047, 3,130,048, 3,130,049, 3,102,809, 3,184,310, 3,188,210, and 3,180,733, West German Patent 938,233, and SPIE Proceedings, vol. 631, page 210, vol. 1,672, page 231 (1992), vol. 1,672, page 262 (1992) and vol. 1,925, page 227 (1993) that the use of a photosensitive material having hydroxyl group in its molecule can provide a resist having a high contrast and a high resolving power.
Selectively esterified photosensitive materials each having three aromatic rings (formulae (III) to (V)) are proposed in SPIE Proceedings, vol. 1,672, pp. 262-272. ##STR1## wherein D represents 1,2-naphtoquinonediazide-5-(and/or -4-) sulfonyl group.
However, these photosensitive materials leave something to be desired in the reduction of dependence on film thickness, though providing a sufficiently enhanced resolving power.
Further, JP-A-2-19846 and JP-A-2-103543 disclose diesterification products of polyhydroxy compound having, e.g., the following specific structural formulae (VI) to (VIII). As a result of the examination of these diester photosensitive materials for dependence on film thickness, it was found that these photosensitive materials, too, leave something to be desired. ##STR2##
Moreover, 1,2-naphthoquinonediazide-5-(and/or -4-) sulfonyl chlorides of compounds having the following structural formulae (IX) and (X) disclosed in U.S. Patent 5,178,986, JP-A-57-63526, and JP-A-62-10646 leave something to be desired in the reduction of dependence on film thickness. ##STR3## wherein R.sub.16b to R.sub.18b, R.sub.23b, R.sub.24b, R.sub.26b, and R.sub.28b, may be the same or different and each represent a hydrogen atom, a halogen atom, an alkyl group, an alkoxy group, an aryl group or a nitro group; and R.sub.19b to R.sub.22b R.sub.25b, R.sub.27b, and R.sub.29b, each represent a hydrogen atom or an alkyl group.
It has heretofore been quite unknown how the composition of resist materials should be formulated to reduce the dependence on film thickness, making it possible to provide a high resolving power independent of film thickness.
With the recent rise in the integration of semiconductor circuits, the demand for the elimination of particles produced from positive-working photoresists has grown more and more. In the field of semiconductor circuits, as the so-called "1/10 rule" says, particles having a size of not less than 1/10 of the smallest line width of device affect the yield of semiconductor circuits (see "Ultraclean Technology", vol. 3, No. 1, page 79 (1991)).
In order to eliminate these particles, some schemes such as use of a filter having a pore diameter as small as 0.1 .mu.m or 0.05.mu.m have been made. These schemes can help eliminate particles during the preparation of resists.
However, even if there are few particles during the preparation of resists, particles can often occur more and more with time. The occurrence of particles with time is mostly attributed to 1,2-quinonediazide photosensitive materials. In order to improve the age stability of resists, various schemes have been made.
For example, the use of a photosensitive material in which the hydroxyl groups in polyhydroxy compound is partially acylated or sulfonylated (JP-A-62-178562), the use of a mixture of 1,2-naphthoquinonediazide-4-sulfonic ester and 1,2-naphthoquinonediazide-5-sulfonic ester (JP-A-62-284354), the use of a thermally modified 1,2-naphthoquinonediazide photosensitive material (JP-A-63-113451), the reduction of residual catalyst in photosensitive material (JP-A-63-236030), the synthesis of a photosensitive material in the presence of an anion exchange resin (JP-A-63-236031), the mixing of a photosensitive material with a solution having a high power of dissolving the photosensitive material (JP-A-61-260239, JP-A-1-293340), etc. have been heretofore attempted.